High temperature drop-off of a substrate

ABSTRACT

A substrate to be processed in a high temperature processing chamber is preheated to avoid the problems associated with thermal shock when the substrate is dropped onto a heated susceptor. Preheating is effected by holding the substrate over a susceptor maintained at or near the processing temperature until the temperature of the substrate approaches the processing temperature. Thus, wafer warping and breakage are greatly reduced, and wafer throughput is improved because of time saved in maintaining the susceptor at constant temperature without cool down and reheat periods.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to substrate transport intoprocessing chambers such as semiconductor chemical vapor depositionreactors, and, more particularly, to a method for increasing substratethroughput and reducing loss of product by reducing thermal shock to thesubstrate, which can cause breakage and poor quality of the depositedfilm.

[0003] 2. Description of the Related Art

[0004] In the manufacture of integrated circuits, semiconductorsubstrates, or wafers, are often processed by chemical vapor deposition.Components of chemical vapor deposition systems include a reactionchamber that is configured to facilitate the controlled flow of areactant gas and a wafer holder, which is commonly referred to in theart as a “susceptor,” for supporting and heating the wafer duringprocessing. To facilitate automated processing, a robotic arm with awafer handler, or end effector, on the end is employed to place a waferonto the susceptor for processing and to remove it from the reactorafter processing.

[0005] In high temperature film deposition or annealing processes, awafer must be heated to a predetermined temperature after it isintroduced into a processing chamber. For example, in epitaxy processes,the temperature is typically around 1090° C. to 1190° C. This waferheating can be effected by conduction through direct contact with aheated susceptor, or by radiation through the use of heating lamps.

[0006] In current chemical vapor deposition epitaxial reactors, thewafer is typically at room temperature when it is loaded into theprocess chamber, which is still at a much higher temperature, perhaps ashigh as 900° C. When the substrate is dropped onto the susceptor, bothsubstrate and susceptor experience thermal shock. As its mass is so muchless than the susceptor's, the substrate's shock is more significant.The thermal effect experienced by the substrate usually variesthroughout the extent of the substrate, that is to say that there arelarge thermal gradients, which may be difficult to mitigate. This canlead to substrate warping and breakage and also to poor uniformity andquality of deposited films.

[0007] The susceptor also experiences thermal shock, which is repeatedwith wafer after wafer, and can ultimately reduce the working lifetimeof the susceptor. Usually the susceptor is prepared to receive a waferby cooling to a temperature much less than the process temperature inorder to reduce the thermal shock. Then, once the wafer is in place, thesusceptor must be reheated to the process temperature before processingcan proceed. This repeated temperature cycling, solely for the purposeof wafer transfer, slows wafer throughput, so most manufacturerscompromise by making only small temperature reductions during wafertransfer. This results in less thermal shock but does not completelysolve the problem.

[0008] There has been some experimentation in the past by customers ofthe assignee of the present invention in which the heaters for thesusceptors were de-energized while a wafer was held above the susceptorfor a short interval, before being deposited on the susceptor. Whilethis pre-heating of the wafer reduced shock to the wafer, there wasstill some warpage of the wafer and the susceptor temperature wasdropping during the delay. The temperature when the handler wasintroduced to the process chamber was probably in the range of about850° C. to about 900° C., the temperature of the susceptor and the waferwas probably below 850° C. when the wafer was deposited on thesusceptor. Also, heat damage to the handler construction limited thetemperature that could be maintained.

[0009] There is a clear need for a method of wafer exchange in hightemperature process chambers that reduces the thermal shock experiencedby both the wafer and the susceptor without adversely affecting processthroughput, and instead enhancing throughput.

SUMMARY OF THE INVENTION

[0010] If the wafer can be preheated to a temperature at or near theprocess temperature before being put on the susceptor, many benefits areaccrued. Wafer throughput is increased because no additional time isneeded to cool down and subsequently reheat the susceptor during wafertransfer. The temperature remains more uniform because the susceptorremains at or close to the process temperature instead of cyclingthrough cooling and re-heating steps. There is reduced wafer breakage,resulting in less downtime. There is less wafer warping, resulting inbetter temperature consistency across the wafer and, therefore, betterdeposited film quality and uniformity. Consumable components, such assusceptors, undergo less thermal shock, thus prolonging their lifetimesand reducing the cost of operating the process equipment.

[0011] In general, an embodiment of the current invention comprises asubstrate holder configured to support a single substrate for processingin a high temperature chamber. Processing methods that can use theapparatus and method recited herein include chemical vapor deposition,epitaxy, rapid thermal processing, etching, annealing, etc. A substratehandler can pick up an unprocessed substrate, put it on the substrateholder for processing and remove it after processing is complete.Heating is effected by a plurality of heating elements that arepositioned to heat the substrate holder, and a control system maintainsthe substrate holder at a high temperature, such as more than 900° C.,during wafer transfer. It is preferred that, while holding anunprocessed substrate, the substrate handler pause in a position closeto and above the substrate holder for a period of time that allows thewafer to preheat. Preferably, the heating elements are radiant lampsthat heat the wafer from above while the hot susceptor is heating thewafer from below, and thus minimize warpage of the wafer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 shows a high temperature process chamber wherein asubstrate is positioned over a heated susceptor during a preheat periodof the method described herein.

[0013]FIG. 2 is a flow chart explaining the steps involved in apreferred embodiment of the invention.

[0014]FIG. 3 is a graph of temperature as a function of time for a waferduring the steps of transport into the chamber, preheating, processingand removal from the chamber according to an embodiment of the currentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0015] A preferred embodiment of the current invention is described withreference to FIG. 1 for a substrate, particularly a silicon wafer 10,transported by an end effector 12 in a cold wall chemical vapordeposition chamber 14 formed by quartz chamber walls. Details about thechemical vapor deposition chamber of the preferred embodiment aretreated in pending U.S. patent application Ser. No. 09/495,765, which isincorporated by reference herein. Heating elements 16 that heat thesubstrate support or susceptor 18 are shown below the chamber. Detailsabout the susceptor can be found in U.S. Pat. No. 6,068,441, issued May30, 2000, which is included by reference herein. Additional heating ofthe susceptor is effected by radiant heating lamps 20 above the processchamber. In another embodiment, the susceptor can also be heated byresistive heating elements within the substrate support. Gas tubes 22attached through the wall of the process chamber 14 provide a path forflow of process gas and purging gas into the chamber 14. Although theapparatus described above is a chemical vapor deposition chamber, themethod described herein applies to other high temperature processchambers as well.

[0016] In a preferred embodiment of the method described herein, theprocess chamber 14 and the susceptor 18 are maintained at, or very nearto, the process temperature, preferably greater than 900° C., morepreferably greater than 1000° C. In the illustrated epitaxial chamber,the process temperature can be as high as 1190° C.

[0017] The wafer 10 is introduced into the process chamber 14 on an endeffector 12, which may be a paddle with a fork-like configuration thatextends beneath a portion of the wafer and leaves a substantial lowerwafer surface (>60%) exposed to the susceptor 18. In this case, the endeffector preferably holds the wafer by its edges. In the illustratedembodiment, however, the wafer is introduced on a Bernoulli wand, whichuses gas flow to create a low pressure above the wafer to hold the waferfrom above without making direct contact with the wafer upper surface. Asuitable Bernoulli wand is described in further detail in U.S. Pat. No.5,997,588, issued Dec. 7, 1999. Another version described in U.S. patentapplication Ser. No. 09/006,325, filed Jan. 14, 1998, has only quartzcomponents that extend into the process chamber, and hence can withstandhigh process temperatures. Both documents are incorporated herein byreference. The Bernoulli wand can be made of material substantiallytransparent to radiant energy, so that it does not prevent radiantenergy from the lamps above from reaching the wafer. Alternatively, theBernoulli wand can be made of material that absorbs radiant energy, sothat the wand heats also and re-radiates or conducts heat to the wafer10.

[0018] The wafer 10 is held above the susceptor 18 by an end effector12, for a length of time determined by a temperature controller 17,preferably until the wafer 10 reaches within about 200° C. of thetemperature of the susceptor 18, and more preferably within about 100°C. of the susceptor temperature. As noted above, the susceptor 18 ispreferably maintained above 900° C. and most preferably at the processtemperature by the temperature controller 17. Preferably, the preheat ofthe wafer 10 is through the direct use of radiant heating lamps 20 tothe upper surface of the wafer and to the lower surface from thesusceptor.

[0019] During the wafer preheat, the purging gas flow rate through thegas tube 22 may be reduced below the normal flow rate used to purge thechamber of process gas. The purging gas flow rate is preferably reducedto 5-10 standard liters per minute. This is done to reduce cooling ofthe wafer from rapid gas flow during the preheat period. Of course, gasthrough the Bernoulli wand is maintained at whatever level is needed tohold the wafer. Thus, reduced flow rate is more significant for apaddle-type wafer handler.

[0020] After a preheat period, the wafer 10 is put on the susceptor 18with minimal thermal shock, and the wafer 10 is ready for processing.The selected preheat period depends upon the starting temperature andthe susceptor idling temperature. In the illustrated embodiment, where a“cold” wafer, at less than about 100° C., more typically less than 50°C., is introduced into the chamber while the susceptor idles at greatthan 900° C., the preheat period is at least 10 seconds, preferablygreater than about 15 seconds.

[0021] The flow chart of FIG. 2 summarizes the steps involved ineffecting wafer preheat according to a preferred embodiment of thecurrent invention. In the first process step 100, the end effector picksup the next unprocessed wafer from a wafer cassette or other storagearea and moves it into the process chamber. In step 110, which isoptional, the purging gas flow rate is preferably decreased to reduceany cooling effects on the wafer as it preheats. Pausing over thesusceptor, which is maintained at or close to the process temperature,the end effector holds the wafer during the preheat period 120. Whenpreheating is complete, the wafer in step 130 is put on the susceptor,and the processing step 140 is performed. When the wafer processing iscomplete, the susceptor still remains at or close to the processtemperature as step 150, and purging gas begins to flow in step 160. Theend effector removes the processed wafer to storage as step 170. Thecycle begins again with step 100, loading of the next unprocessed wafer.

[0022] The temperature benefits of wafer preheating according to thepreferred embodiment can be further understood with reference to thegraph of FIG. 3. The wafer is typically at room temperature when it ispicked up from the cassette by the end effector, as indicated as step100 of the flow chart. The wafer is moved into a position over thesusceptor, which is at the process temperature of about 900° C. orgreater, and held for the preheat period 125 in accordance with step120. By the end of the preheat period 125, the wafer has reached atemperature of about 700° C., or about 200° C. below the processtemperature. The wafer is put on the susceptor in step 130, and reaches900° C. within a few seconds. The wafer could be heated further beforebeing placed on the susceptor, but the temperature difference of about200° C. is small enough to ensure that thermal shock will be negligible.Temperature is shown as constant during processing on the graph of FIG.3, but temperatures vary for particular processes and may includemultiple steps at different temperatures. Preferably at least the firstprocess step is at or above the susceptor idling temperature. The graphillustrates a preheat period 125 of about 25 seconds, but that too willvary with different reactions. After processing, the wafer is picked upfor removal by the end effector.

[0023] Advantageously, the preheat approach reduces wafer warping andbreakage, which results in better quality processing and lessunscheduled down time to remove wafer fragments from the equipment.Wafer throughput is also increased because time is no longer spent ontemperature cycling, i.e., cooling down and reheating, the susceptorduring wafer transfers. The susceptor can be maintained at the sameprocess temperature all the time.

We claim:
 1. An apparatus for processing substrates comprising: asubstrate support; a substrate handler to place a substrate on thesupport and to remove the substrate from the support; one or moreheating elements positioned to heat at least the substrate and thesubstrate support; and a control system configured to cause the handlerto hold the substrate above the substrate support for a preheat periodbefore placing the substrate on the support, while maintaining a supporttemperature of greater than about 900° C. during the time the substrateis being moved on or off the support.
 2. The apparatus of claim 1wherein the heating elements include radiant heat lamps.
 3. Theapparatus of claim 1 wherein the heating elements include a resistiveheating element in the substrate support.
 4. The apparatus of claim 1wherein said heating elements include a plurality of radiant heatinglamps which are positioned above the substrate support, and said handleris configured to move above the substrate support and below the lamps,said control system during said preheat period is configured to causethe lamps to heat the upper side of said substrate during said preheatperiod while said substrate support heats a lower side of the substrate.5. The apparatus of claim 4 wherein said substrate handler is aBernoulli wand which is configured to hold the substrate from above bycreating a low gas pressure above the wafer without contacting the uppersurface of the wafer, and said wand is substantially transparent toradiant energy such that the substrate can be heated from above by saidlamps.
 6. The apparatus of claim 4 wherein said substrate handler is aBernoulli wand which is configured to hold the substrate from above bycreating a low gas pressure above the wafer without contacting the uppersurface of the wafer, and said wand absorbs radiant energy such that thewand and the substrate can be heated by said lamps.
 7. The apparatus ofclaim 4 wherein said substrate handler includes a paddle to support asubstrate from below while maintaining a substantial portion of thelower surface of the substrate exposed to the substrate support duringsaid preheat period.
 8. The apparatus of claim 4 wherein said substratehandler comprises an end effector that holds the substrate by substrateedges.
 9. The apparatus of claim 4 including gas tubes connected toprovide purging gas to a chamber enclosing the substrate support, andsaid control system being configured to reduce the flow of purging gasinto the chamber during the preheat period.
 10. A method of processing asubstrate on a heated substrate support in a high temperature processchamber comprising: using a substrate handler to move the substrate intothe process chamber to a position above the substrate support; holdingthe substrate above the substrate support for a substrate preheatperiod; maintaining the substrate support at a processing temperatureduring the substrate preheat period; and putting the substrate on thesubstrate support at the completion of the substrate preheat period. 11.The method of claim 10 wherein the substrate has a temperature less than100° C. as it moves into the chamber.
 12. The method of claim 10 whereinthe substrate has a temperature less than 50° C. as it moves into thechamber.
 13. The method of claim 10 wherein the processing temperatureis greater than 900° C.
 14. The method of claim 10 wherein theprocessing temperature is greater than 100° C.
 15. The method of claim10 wherein the preheat period is longer than 10 seconds.
 16. The methodof claim 10 wherein, during the preheat period, the substrate is heatedto a temperature of about 200° C. or less below the temperature of thesubstrate support.
 17. The method of claim 10 wherein, during thepreheat period, the substrate is heated to a temperature of about 100°C. or less below the temperature of the substrate support.
 18. Themethod of claim 10 wherein during the preheat period, the substratesupport and the substrate are heated by means of radiant heating lamps.19. A method of improving the throughput time of a high temperaturesubstrate processing apparatus comprising: providing a substrate supportwithin a processing chamber; providing radiant heating elements spacedabove the substrate support; moving a substrate into the chamber;positioning the substrate at a location between the radiant heatingelements and the substrate support; preheating the substrate with heatfrom the heating elements and from the heated substrate support;maintaining the substrate support at a process temperature while thesubstrate is preheating; and depositing the preheated substrate onto thesubstrate support.
 20. The method of claim 19 wherein preheating thesubstrate continues until the temperature of the substrate is within200° C. of the substrate support
 21. The method of claim 19 whereinpreheating the substrate continues until the substrate is within 100° C.of the substrate support.
 22. The method of claim 19 wherein the processtemperature is greater than 900° C.
 23. The method of claim 19 whereinthe process temperature is greater than 1000° C.
 24. The method of claim19, further comprising processing the substrate at the processtemperature.
 25. The method of claim 19, further comprising removing thesubstrate from the substrate support after processing and moving thesubstrate out of the chamber, while maintaining the substrate support atthe process temperature.